All-optical, optically addressable liquid crystal-based light valve employing photoswitchable alignment layer for high-power and/or large aperture laser applications

ABSTRACT

A beam shaping system including an all-optical liquid crystal beam shaper, the beam shaper including a photoswitchable alignment material including at least one of a PESI-F, SPMA:MMA 1:5, SPMA:MMA 1:9, ora SOMA:SOMA-p:MMA 1:1:6 material, at least some of the liquid crystals of the beam shaper including at least one of a phenylcyclohexane, cyclo-cyclohexane, or a perfluorinated material.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-EE0006033,DE-NA0001944 and DE-NA0003856 awarded by the Department of Energy. Thegovernment has certain rights in the invention.

RELATED FIELDS

Liquid crystal beam shaping systems for laser applications.

BACKGROUND

Liquid crystals (LC's) have anisotropic optical properties that makethem ideal materials from which to construct either passive or activedevices that offer polarization, phase, or intensity control. ThoughLC's are commonly associated with the display industry, they are usefulin a wide variety of other applications, including in various lasersystems for spectral and polarization control of the laser beam. Forexample, LC circular polarizers and wave plates have been key componentsin the near-infrared portion of the 351-nm, 1-ns, 40-TW OMEGA laser atthe Laboratory for Laser Energetics (LLE) for over 30 years. The laserinduced damage threshold (LIDT) of the material (or optical component)is a key factor in determining the suitability and performanceparameters for most optical materials incorporated in such lasersystems. Because the utilization of lasers is continuously expandinginto an increasing number of applications, there is a growing need foroptical components suitable for higher average power and/or peakintensity systems. As the constituent optical materials and opticalcomponents represent a limit (damage threshold) on how much energy theycan handle, increasing the power output of laser systems requires anaccompanying increase in their clear aperture. An additional growingneed is associated with attaining a higher quality spatial distributionof the laser energy within the laser beam profile, including the abilityto control the spatial distribution of the beam amplitude and/or phase.Optical devices that can offer this capability, typically referred togenerically as “light valves”, must be able to withstand the transmittedpower of the laser beam while maintaining performance and beam profilecharacteristics for the application-specific time durations.

Recent work has highlighted the potential of LC electrically-biasedoptically-addressable light valve (OALV) technology to advance additivemanufacturing applications (3D printing) by allowing for a continuouslyvariable beam profile to print entire layers of metal powdersimultaneously. Such an approach will expedite the manufacturingprocess, reduce cost, and improve as built material properties.

SUMMARY

This patent describes examples of all-optical, optically addressable,liquid crystal-based light valves for high-power, large aperture, andother laser applications. These light valves employ specific classes ofmaterials and optical arrangements allowing their use with high averagepower and/or peak intensity laser applications, such as for lasersystems for additive manufacturing and directed energy applications, andfor other laser applications. The approach uses an all-optical design(no conductive oxide electrodes, electrical interconnects, etc.) basedon unique photoswitchable alignment layers and LC materials.

The photoalignment layers and LC materials used in the devices areexceptionally resistant to damage from high-intensity laser energy andprovide the ability to reproducibly write, store, and erasehigh-resolution optical patterns with minimal loss of resolution andcontrast after multiple write/erase cycles. The LC materialsincorporated into the device are preferably saturated materials (i.e.,those that contain a minimum of carbon-carbon double bonds), even morepreferably fully saturated materials. The combination of optical valvedesign and specific materials/classes of materials described in thispatent are advantageous for several reasons:

a) High damage threshold: the photoswitchable alignment materials andsaturated LC materials described in this patent provide optimized damagethresholds that nears, meets, or in some cases exceeds that of theconventionally-polished fused silica substrates used in these devices,thus reaching a fundamental upper limit.

b) Low absorption at the operational (laser) frequency: due to thenanometer-scale thickness of the alignment layers, any significantenergy deposition to the device by the laser beam would occur only viaLC material absorbance. As discussed below, the saturated LC materialsidentified below provide a blue shifted absorption spectrum thatsupports absorption free operation through the visible and near infraredspectrum, extending in the near ultraviolet region (subject toconsideration of wavelengths that would write/erase/rewrite thephotoalignment layer).

c) Stability of the molecular orientation/optical performance duringoperation: the adaptation of fully saturated LC materials along with thenovel photoalignment layers described in this disclosure provide uniquebenefits. Specifically, spiropyran and spiroxazene materials arechemically more stable than azobenzene materials and produce strong LCordering extending over LC device path lengths of at least 24 μm. Forsaturated LC materials, the lack of a delocalized π electron systemminimizes the potential coupling with the intense optical field of thelaser, completely eliminating third-order nonlinear opticalreorientation of the LC molecular axis which, although known to occur inunsaturated LC materials, is slow (on the order of several seconds) andat best would produce some loss of alignment, which could be correctedeasily by refreshing the written pattern with another optical writecycle. The combination of strong, rewritable LC surface anchoring with anearly nonexistent LC reorientational response to strong incident laserfields allows the desired optical patterns to be written and maintainedfor a duration of time that could meet or exceed therequirements/specifications for the intended applications.

d) Ability to change optical pattern while maintaining optical quality:the novel photoalignment layers disclosed in this patent provide theability to reproducibly write, store and erase high-resolution opticalpatterns with minimal loss of resolution and contrast after multiplewrite/erase cycles, and resistance to image-sticking and burn-in.

The following sections provide background on certain fundamental issuesapplicable to the light valves described in this patent.

Light Valve Design

The ability to produce precise spatial shaping of the amplitude or phaseof an incident high-energy laser beam is a key requirement for a numberof applications in optics and photonics. One example is in high-energylaser systems employing large-scale 1053 nm Nd:glass beamlines to eitherpre-compensate for spatial-gain variations or maximize the energyextraction using apodized high-order super-Gaussian beams. Laser beamshaping in such systems has employed binary devices composed ofdistributions of opaque pixels with transmission equal to either 0 or100%. These devices are typically in the form of a metal film depositedon a transparent glass substrate which, when positioned appropriately inthe laser beam, produce a continuous beam profile after far-fieldFourier filtering and re-imaging.

Metal-mask beam shapers are prepared using standard photolithographicprocessing techniques widely employed in the semiconductor fabricationindustry and are relatively inexpensive to make, but have two keylimitations: (1) a new mask must be generated if it is desired to changethe shaping pattern (i.e., real-time manipulation of the spatialamplitude distribution is not possible); and (2) because the metal maskcontrols the beam shaping profile by spatially-distributed absorption ofthe near-IR laser energy by the metal pixels, the resistance of thesedevices to damage by the incident laser energy is very small (˜200-700mJ/cm2 at 1053 nm, 1 ns pulse). As a result, these devices are limitedto use only in low-fluence areas of the laser system.

An alternative method utilizes LC materials in either active(electric-field-driven control of the optical characteristics of thedevice) or passive (fixed optical properties, e.g. Dorrer et al., 2011).Such LC materials and devices have demonstrated significant potentialfor both polarization control and beam-shaping of relatively high-powernear IR lasers, and have an extended track record of proven performancein various locations in both the 60-beam, 40-TW OMEGA and 4-beam, 4petawatt Nd:glass laser systems in the Omega Laser Facility at theUniversity of Rochester's Laboratory for Laser Energetics (LLE).

Specific advantageous properties of LC-based devices include scalabilityto apertures of 200 mm or larger, cost effectiveness, high opticalquality with low loss and high contrast, broad angular tolerance,ability to tune optical properties through LC material composition and,most importantly, a relatively high intrinsic laser damage resistance,as demonstrated with current generation materials. However, the inventorhas discovered that, by suitable tailoring of the LC material'smolecular structure, the damage threshold can be increased significantlybeyond current benchmark values, not only in the near infrared spectralrange but also through the near ultraviolet spectral range (subject toconsideration of the wavelengths that would write, rewrite, and/or erasethe pattern written into the photoalignment layer).

Alignment Layers and Photoconductive Films

Alignment layers in LC devices serve to establish a uniform alignmentdirection of the LC material throughout the bulk of the LC fluid layer,which significantly improves optical contrast and minimizes defects. TheLC molecules at the surface of the alignment layer are tightly bound ina particular orientation determined by the boundary conditions and theelastic deformation energy of the LC molecules, which in turn dependsupon the particular surface treatment employed. Commercially availableLC devices are typically fabricated by rubbing (buffing) the alignmentlayer in a particular direction before device assembly to establish themolecular alignment direction of the LC. Typically a polyimide alignmentlayer is employed, although other polymers (e.g., Nylon 6/6) have alsobeen used.

Photolithographic patterning of UV photosensitive LC alignment layerswith linearly polarized UV light has been used to fabricate passive LCbeam shaper devices with spatially varying molecular orientations. Thesephotoalignment materials have high near IR laser damage thresholds.Coupled with the ability to generate an almost infinite variety ofbinary and gray-scale apodization and beam-shaping profiles by thephotoalignment process, the high laser-damage threshold, ease inprocessing flexibility, and the ability to scale to large aperturesthrough conventional contact photolithography techniques make this typeof device ideal for passive beam-shaping applications in high-fluenceareas of high-power laser systems. A significant short-coming of thistype of LC beam shaper for many applications is that its shaping profileis static; the photo-patterned LC pixel orientations cannot be changedor erased once they have been written, which necessitates fabrication ofa new device for each desired beam shaping profile.

Active LC beam shaper devices provide “real-time”, spatially-distributedamplitude and phase modulation of laser beams using matrix-addressed LCelectro-optical spatial light modulators (SLM's). One example of thistype of device is the commercially available liquid crystal-on-silicon(LCOS) reflective SLM's for programmable beam shaping at high spatialresolution (typically 600×792 pixels) in low-fluence areas ofhigh-peak-power lasers such as the 4 petawatt OMEGA EP laser and theMulti-Terawatt (MTW) laser at LLE. Each 10 μm×10 μm LCOS-SLM pixel canapply a programmable phase change to the beam, while transmission iscontrolled by adjusting the SLM's phase-modulation depth. For real-timecontrol of the LCOS SLM device, the SLM image plane is captured using anear-field camera and the beam wave front is measured with a wave frontsensor. The 2-D near-field and wave front profiles are used to provideclosed-loop feedback control of the SLM. A computer program iterativelycontrols the SLM based on the measured 2-D profiles to achieve thedesired profile.

Matrix-addressed electro-optical LCOS-SLM devices allow for considerableflexibility for real-time beam shaping but the requirement for patternedmetal-oxide conductive coatings on the inner cell surfaces makes deviceassembly complicated, and, most importantly, significantly reduces theamount of laser fluence that the device can handle before incurringpermanent and catastrophic modification (damage). It has been recentlyshown that the onset of damage (threshold) is determined by theabsorption and heating of a nanoscale region of a characteristic sizereaching a critical temperature, which is applicable to indium tin oxide(ITO) films (exhibiting laser damage threshold at 1054 nm, 2.5 ns ofabout 250 mJ/cm²) and also to conductive wide band-gap semiconductors(damage threshold on the order of 5 J/cm²).

Electrically-biased, transmissive single pixel optically addressed LClight valve technology have also been employed for active beam shaping.This LC device contains continuous (un-patterned) metal-oxide electrodeson the both of the cell's inner substrate surfaces and a secondarybismuth silicon oxide (BSO) photoconductive film on top of one of theconductive-oxide coated surfaces. An AC bias voltage adjusted to beslightly below the threshold voltage for LC reorientation is applied tothe indium-tin oxide (ITO) conductive coatings and the desiredbit-mapped, beam shaping image is projected onto the BSO photoconductivefilm using light from a 470-nm light-emitting diode (LED) source. Thespatially distributed UV intensity induces a corresponding spatiallydistributed localized DC voltage that, when combined with the AC biasvoltage across the ITO electrodes, causes the LC molecules in theilluminated areas to reorient and modulates the polarization of the1053-nm incident laser beam.

Although this electrically-biased OALV approach significantly simplifiesboth device fabrication and operation as compared to thematrix-addressed LCOS-SLM device, it nevertheless suffers from the samefundamental limitation for applications in high-power laser beam shapingthat are germane to electro-optical LC devices: the inherent low near IRlaser-damage threshold of the internal conductive coatings required inorder for the electric field to penetrate the LC material and inducemolecular reorientation.

LC Materials

Liquid crystal materials exhibit properties of both fluids (inability tosupport a shear stress) and crystalline solids (ordered molecularorientation) and, in general, they partially or fully lack positionalmolecular order. The two main classes of LC's, (thermotropic andlyotropic) are distinguished by the physical parameters that controlover what conditions the liquid crystalline phase will appear (changingtemperature or changing solution concentration, respectively). The mostcommon thermotropic LC materials consist of either rod-like (calamitic)or disk-like (discotic) molecules. Calamitic LC materials can exhibitseveral different mesophases (e.g., nematic, smectic, and cholestericphases) that differ depending on the molecular structure of the materialand the degree of order of the mesophase. The directional molecularordering characteristic in these mesophases gives rise to usefulproperties (e.g., optical and dielectric anisotropy) and are thusparticularly suitable for optical applications.

Both the magnitude of the refractive indices and the absorption spectracharacteristics are of fundamental importance in LC materials suitablefor optical applications and are determined by the molecular andelectronic structure of the material. The majority of liquid crystalsreported to date consist of either saturated cyclohexane rings (in whichno double bonds or π electrons are present) or unsaturated phenyl rings(which contain delocalized π electrons); the latter make the largestcontribution to the optical absorption and refractive indices of the LCmolecules. Cyclohexane rings contain only σ-electrons and theirelectronic transitions are blue shifted compared to those of unsaturatedmolecules.

Laser-induced damage is determined by the formation of an observablematerial modification, which requires the deposition of laser energyinto the material. The energy-coupling mechanisms are largely dependenton the electronic structure of the material, the presence of absorbingdefects structures, and the associated excitation (laser) parameters. Inthe case of LC materials, defects may be intrinsic (such as related tomolecular orientation and domain boundaries) or extrinsic (impurities,substrate defects or inclusions).

Specific Issues for High Power Laser Applications

The development of addressable optical light valves that exhibitenhanced performance for high average power and/or peak intensity laserapplications involves a number of important technical issues to beaddressed that relate to the properties of the constituent materials.These include:

-   -   a) High damage threshold to enable operation at increased laser        fluences, which dictates that individual components and        materials used in the device exhibit high damage resistance.    -   b) Low absorption at the operational (laser) frequency to reduce        excessive heating of the device with subsequent changes in the        optical properties of the switching medium.    -   c) Stability of the molecular orientation/optical performance        upon repeated exposure to elevated temperatures and the        polarized electric field of the laser.    -   d) Ability to reproducibly write and erase optical patterns in        application-relevant time scales while maintaining optical        quality (i.e. be resistant to image-sticking or burn-in).

In one example, a beam shaping system includes: (a) a liquid crystalbeam shaper, the beam shaper including a photoswitchable alignmentlayer, the photoswitchable alignment layer having at least one of aPESI-F, SPMA:MMA 1:5, SPMA:MMA 1:9, or a SOMA:SOMA-p:MMA 1:1:6 material;and (b) an optical writing and erasing sub-system configured to write,erase, and rewrite a plurality of optical patterns in thephotoswitchable alignment layer.

In some instances, the photoswitchable alignment layer may have anN-on-1 laser induced damage threshold of 80-100 J/cm² at 1053 nm.

In some instances, at least some liquid crystals of the beam shaper mayinclude at least one of partially saturated liquid crystals, fullysaturated liquid crystals, partially fluorinated liquid crystals, orperfluorinated liquid crystals.

In some instances, at least some liquid crystals of the beam shaper maybe at least one of a phenylcyclohexane, cyclo-cyclohexane, or aperfluorinated material.

In some instances, the liquid crystal beam shaper may be an all-opticalliquid crystal beam shaper.

In some instances, the liquid crystal beam shaper may include: (i) afirst transparent glass substrate including a first coating on an innersurface of the substrate, the first coating being the photoswitchablealignment layer; (ii) a second transparent glass substrate including asecond alignment coating on an inner surface of the substrate, thesecond alignment coating being either a buffed alignment layer or alayer that has been permanently oriented using polarized UV light; (iii)the liquid crystals of the beam shaper located between the first andsecond substrates; and (iv) a polarizer.

In some instances, the beam shaping system is configured such thatwriting an optical pattern into the photoswitchable alignment layercauses a localized change in configuration of the liquid crystals, suchthat a laser beam passing through the liquid crystal beam shaperundergoes a localized change in polarization state.

In some instances, the polarizer is configured to pass or rejectportions of the laser beam depending on the localized change inpolarization state.

In some instances, the first and second transparent glass substrates arefused silica.

In some instances, the second alignment coating is an alignment coatinghaving a fixed alignment state.

In some instances, the second alignment coating is a buffed Nylon 6/6,ROLIC ROP 203/2CP, Polymer 3, or a LIA-01 material.

In some instances, the optical writing and erasing sub-system includes:(i) a UV light source and a spatial light modulator configured to writean optical pattern on the photoswitchable alignment layer; or (ii) araster-scanned UV laser source configured to write the optical patternon the photoswitchable alignment layer.

In some instances, the optical writing and erasing sub-system isconfigured to erase the written optical pattern by application of UVlight having a different polarization than the UV light used to writethe optical pattern, or by application of visible light.

In another example, a beam shaping system may include: (a) a liquidcrystal beam shaper, the beam shaper including a photoswitchable polymercommand surface, the photoswitchable polymer having an N-on-1 laserinduced damage threshold (J/cm²) of 80-100 at 1053 nm; and (b) anoptical writing and erasing sub-system; the photoswitchable polymercommand surface and the optical writing and erasing sub-systemconfigured to write, erase, and rewrite a plurality of optical patternsin the photoswitchable alignment layer.

In some instances, at least some liquid crystals of the beam shaper arepartially saturated liquid crystals, fully saturated liquid crystals,partially fluorinated liquid crystals, or perfluorinated liquidcrystals.

In some instances, the liquid crystal beam shaper is an all-opticalliquid crystal beam shaper.

In another example, a beam shaping system may include: (a) anall-optical liquid crystal beam shaper, the beam shaper having: (i) afirst transparent glass substrate having a first coating on an innersurface of the substrate, the first coating comprising a photoswitchablealignment material comprising at least one of a SPMA:MMA 1:5, SPMA:MMA1:9, or a SOMA:SOMA-p:MMA 1:1:6 material; (ii) a second transparentglass substrate having a fixed alignment coating on an inner surface ofthe substrate; (iii) liquid crystals between the first and second glasssubstrates, wherein at least some liquid crystals of the beam shaper area phenylcyclohexane, cyclo-cyclohexane, or a perfluorinated material;and (iv) a polarizer; and (b) an optical writing and erasing sub-systemconfigured to write, erase, and rewrite a plurality of optical patternsin the first coating.

In some instances, the beam shaping system is configured such thatwriting an optical pattern into the first coating causes a localizedchange in configuration of the liquid crystals, such that a laser beampassing through the liquid crystal beam shaper undergoes a localizedchange in polarization state.

In some instances, the polarizer is configured to pass or rejectportions of the laser beam depending on the localized change inpolarization state.

In some instances, the optical writing and erasing sub-system includes:(i) a UV light source and a spatial light modulator configured to writean optical pattern on the first coating; or (ii) a raster-scanned UVlaser source configured to write the optical pattern on the firstcoating.

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 shows an example of an optically addressable light valve.

FIG. 2 shows an example of a photoswitchable command surface.

FIG. 3 shows examples of photoswitchable LC alignment materials.

FIGS. 4(A) and (B) show the chemical structure of the poly(estermide)PESI-F.

FIG. 5 shows an example of PESI-F synthesis.

FIG. 6 shows the chemical structure of SPMA:MMA.

FIG. 7 shows an example of SPMA:MMA synthesis.

FIG. 8 shows the chemical structure of SOMA:SOMA-PMMA (1:1:6).

FIG. 9 shows an example of SOMA:SOMA-PMMA (1:1:6) synthesis.

FIG. 10 is a chart of laser induced damage thresholds at 1053 nm forvarious photoalignment materials.

FIG. 11 shows examples of parallel-aligned, single-pixel LC devicesfabricated using the spiropyran and spiroxazane photoswitchablecopolymer alignment layers.

FIG. 12 is a plot of the 1-on 1 and N-on-1 LIDT values for various LCmaterials as a function of the UV absorption edge at various pulselengths. The compound names and brackets identifying the saturated,unsaturated, and mixed materials in (a) apply to (b) through (d) aswell.

FIG. 13 shows N-on-1 LIDT values for saturated and unsaturated LCmaterials plotted as a function of pulse length. The R² values of thefit lines range between 0.90 (E7) and 0.96 (PPMeOB/PPPOB, 1550C, andZLI-1646).

FIG. 14 charts the relative difference between N-on-1 LIDT's ofsaturated and unsaturated compounds as a function of pulse length. Dataare normalized to results obtained for the unsaturated cyanobiphenyl LCmixture E7. At 10 ps there is a substantial increase in the differencebetween the LIDT of the two materials, which suggests a change in themechanism for laser-induced damage.

FIG. 15 is a chart quantifying the difference in intensity at the LIDTobserved between successive test pulse lengths for both saturated andunsaturated materials.

FIG. 16 illustrates certain electronic transitions leading to laserinduced breakdown in LC materials.

FIG. 17 charts LIDT and absorption edge for certain saturated andunsaturated LC materials.

FIG. 18 charts LIDT results for saturated and unsaturated LC materialsirradiated with 527-nm and 1.2-ns pulses.

FIG. 19 charts the N-on-1 average LIDT values for nanosecond pulses atall three wavelengths as a function of each material's absorption edge,which enables direct comparison of the relative differences in LIDTarising from differences in the excitation process.

FIG. 20 charts the average damage intensity for saturated andunsaturated materials for each tested pulse length and wavelengthcondition. The mechanism for laser conditioning appears to have aminimum intensity of ˜5 GW/cm². The data also show a lower bound for thetime scale (<50 ps) during which laser conditioning cannot take place.

DETAILED DESCRIPTION

Optically Addressable Light Valve Design

A major limitation in current OALV designs is the use of conductivecoatings that are known to reduce the damage threshold and increaseabsorption. FIG. 1 schematically shows an example of an opticallyaddressable light valve design that does not require the use ofconductive coatings and that employs a photo-switchable alignment layerinstead.

FIG. 1 shows an all-optical beam shaper using a photoswitchable polymer,or “command surface,” as an alignment coating. Incident low power UVlight from a variety of sources (here, either an Hg/Xe lamp, LED source,or Ar+ laser) provides the “write” illumination to a liquid crystal onsilicon (LCOS) device, whose image plane is focused onto thephotoalignment layer within the beam shaper. Patterns produced on theLCOS are written, erased, and rewritten on the photoalignment layerusing either alternating UV incident polarizations or serialapplications of UV and visible light. Alternatively, patterns can bewritten and erased directly using a raster-scanned polarized UV lasersource or in other manners.

The device shown in FIG. 1 utilizes two different LC alignment layerslocated on each inner substrate surface that is in contact with the LCmaterial. In FIG. 1 , one alignment layer is a buffed nylon 6/6 passivealignment layer. Such materials are currently used as alignment layersfor LC waveplates in OMEGA and are known to have 1053 nm, 1 nslaser-damage threshold in the near-IR of 9-14 J/cm², depending onwhether the coating is buffed or pristine (not buffed), respectively.See (1) Jacobs, S. D., Cerqua, K. A., Marshall, K. L., Schmid, A.,Guardalben, M. J. and Skerrett, K. J., “Liquid-crystal laser optics:Design, fabrication, and performance,” J. Opt. Soc. Am. B 5(9),1962-1979 (1988); (2) Schmid, A., Papernov, S., Li, Z.-W., Marshall, K.,Gunderman, T., Lee, J.-C., Guardalben, M. and Jacobs, S. D.,“Liquid-crystal materials for high peak-power laser applications,” Mol.Cryst. Liq. Cryst. 207, 33-42 (1991). In other examples, a “write-once”photoalignment layer can be substituted for the buffed alignment layer.Options for a passive photoalignment layer include, without limitation,Nylon 6/6 (Poly(N,N′-hexamethyleneadipinediamide)), ROLIC ROP 203/2CP (acinnamate photopolymer available from ROLIC Corp, Switzerland), Polymer3 (a coumarin-based photoalignment layer material), and LIA-01 (anazobenzene photoswitchable alignment layer available from DIC Corp,Japan).

In FIG. 1 , the second substrate contains a photoswitchable “commandsurface” polymer alignment layer that undergoes a reversible change inmolecular shape or orientation when exposed sequentially to low incidentenergy UV or visible light (or UV light with two different polarizationstates). Using any number of imaging techniques (contactphotolithography, laser raster-scanning, or projecting a pattern in theimage plane of an LCOS¬SLM onto the coated surface using a polarizedlight source), spatially distributed alignment states can be written,erased, and re-written into this “all-optical” photoswitchable LC beamshaper, thereby duplicating the behavior of an electro-optical SLM orelectrically-biased OALV without the need for conductive coatings. Asshown in FIG. 1 , either incoherent or coherent UV sources (Hg/Xe lamp,LED, argon-ion, or helium-cadmium laser) can be used to provide theincident write illumination to the LCOS device or, alternatively,directly to the all-optical LC beam shaper if another image generationprocess is desirable (e.g., laser raster-scanning).

Unlike earlier OALV devices, the written state in these photoswitchablealignment layers requires no applied electrical or optical fields toremain stable for extended periods of time (weeks or longer) undernormal ambient conditions, provided that background UV or visible lightintensity remains below the threshold intensity required to change theorientation of the command surface. This switching threshold is afunction of the molecular structure of the command-surface material andcan be controlled by molecular design to be as low or high as necessaryto suppress switching by ambient effects. Write-erase times aredependent on the incident UV energy, and can be as fast as 10 ms.

The pendants on this photoswitchable command surface are switchedoptically between two different alignment states, which in turnredirects the orientation of the LC material in contact with the coatingsurface in response to wavelength of the polarized “write” (UV) or“erase” (visible) incident light. FIG. 2 shows an example of aphotoswitchable command surface including azobenzene pendant groups. Theazobenzene groups, in the elongated trans state (left) cause LCmolecules to adopt an orientation parallel to the azobenzene longmolecular axis, while azobenzenes in the bent cis state (right) switchthe orientations of the LC to a near-parallel orientation to thesubstrate to minimize their free energy. Depending on the molecularstructure of the command surface, the resultant LC reorientation canoccur either out of the substrate plane (top) or in the plane of thesubstrate (bottom). This change in orientation induces a change in thepolarization, phase, or amplitude of an incident optical beam, dependingon the optics in the system.

Although not specifically shown in FIG. 1 or 2 , the outer surface ofthe substrate(s) may include anti-reflection (AR) coatings to minimizeback-reflection and other stray light that could, in some instances,have a negative effect on the pattern writing process. The AR coatingsmay be designed to operate at the wavelength of the write illuminationsource, and, for the sections in which the near IR beam passes throughthe substrate, a second AR coating may be required with propertiesoptimized for the near IR.

For the all-optical near IR LC laser beam shaper (light valve) shown inFIG. 1 to be viable for high-power beam shaping applications, theresistance to laser-induced damage of its components will be animportant consideration. Glass substrates, particularly fused silica,have laser damage thresholds at 1064 nm in excess of 60 J/cm². Inaddition, it has been previously shown that photoalignment materialshave exceptional near IR laser damage thresholds, approaching that ofconventionally polished fused silica [See “Photoaligned Liquid CrystalDevices for High-Peak-Power Laser Applications,” K. L. Marshall, C.Dorrer, M. Vargas, A. Gnolek, M. Statt, and S.-H. Chen, in LiquidCrystals XVI, edited by I. C. Khoo (SPIE, Bellingham, Wash., 2012), Vol.8475, Paper 84750U (invited)]. Furthermore, Marshall et al. reported in2013 the first near IR damage threshold measurements on the commerciallyavailable azobenzene photoswitchable alignment layers PAAD 22, PAAD 27,and PAAD 72 [reference: “Liquid Crystal Near-IR Laser Beam ShapersEmploying Photoaddressable Alignment Layers for High-Peak-PowerApplications,” K. L. Marshall, D. Saulnier, H. Xianyu, S. Serak, and N.Tabiryan, in Liquid Crystals XVII, edited by I. C. Khoo (SPIE,Bellingham, Wash., 2013), Vol. 8828, Paper 88280N]. These materialsexhibited 1053 nm, 1.4 ns laser damage thresholds as high as 66 J/cm².

The ability to reversibly write high-resolution optical patterns into anLC device containing azobenzene photoswitchable alignment layers hasbeen known and investigated for a number of optics and photonicsapplications other than laser beam shaping since the early 2000's. InAugust of 2018, Marshall et al. demonstrated the ability to writeoptical patterns at a resolution of 28.5 line pairs/mm using a UV lightsource and a photolithographic mask into a LC device employingcommercial PAAD 27 azobenzene photoswitchable alignment layers, butseveral problems affecting device operational lifetime were encountered:(1) devices could only be written and erased up to six times beforesignificant resolution degradation was observed, and (2) “imagesticking” (reappearance of multiple patterns from previous exposures)occurs after several sequential patterning cycles. [See “OpticallyAddressable Liquid Crystal Laser Beam Shapers Employing PhotoalignmentLayer Materials and Technologies”, K. L. Marshall, J. Smith, A.Callahan, H. Carder, M. Johnston, and M. Ordway, presented at the SPIEOptics and Photonics Liquid Crystals XXII Symposium, San Diego, Calif.,19-23 August 2018.]

In order for an all-optical LC beam shaper device to realize its fullapplications potential and advance the state of the art for high-powerbeam shaping applications such as additive manufacturing, new photoswitchable alignment layer materials are needed whose molecularstructures and switching mechanisms will provide (1) excellent near IRlaser damage threshold; (2) the ability to reproducibly write, store anderase high-resolution optical patterns; (3) minimal loss of resolutionand contrast after multiple write/erase cycles, and (4) be resistant toimage-sticking and burn-in of previously written patterns. In addition,the LC materials ideally will provide a similarly high damage threshold.

Photoswitchable Alignment Layer Materials for High-Power Laser BeamShaping

The inventor has developed several unique photoswitchable LC alignmentpolymer coatings based on azobenzene, spiropyran and spiroxazanephotoactive pendants. Spiropyrans and spiroxazanes differ fundamentallyfrom the azobenzene photoswitchable coatings in that photoswitchingoccurs due to a reversible photomediated ring opening/closing reactionupon absorption of UV and visible light rather than throughphotomechanical trans-cis isomerization, in which no chemical bonds arebroken.

The generalized schematic diagram for these new materials is shown inFIG. 3 . FIG. 3 shows a photoswitchable alignment material in which thechromophore containing appropriate terminal group (as shown, eitherazobenzene or spiropyran) is tethered to a polymer backbone by aflexible alkyl spacer chain. In FIG. 3 , for photoswitchable LCalignment materials utilizing azobenzene as a chromophore, a reversiblephotomechanical trans-cis photoisomerization is employed. For spiropyran(shown) and spiroxazane chromophores, a photomediated ringopening/closing reaction is employed.

Specifically, three photoswitchable alignment material families weredeveloped:

1. PESI-F

FIG. 4(A) shows the chemical structure of the poly(esterimide) PESI-F.This material is an indolene polymer with azobenzene chromophorespartially incorporated in the backbone and not through a flexibletether. FIG. 4(B) shows how the material switches in response to UV andvisible light. Weglowski et al. (Opt Commun 400, 144 (2017)) reportedthese materials to be useful for fabrication of photochromic diffractiongratings, but, to the best of the inventor's knowledge, it had not beenpreviously known prior to the inventor's discovery to use PESI-F in aphotoswitchable LC alignment layer.

FIG. 5 shows one example of how PESI-F may be synthesized, with anoverall product yield of approximately 10%. [See A. Kozanecka-Szmigielet al., Dyes Pigments 114, 151 (2015).]

2. SPMA:WA (1:5 and 1:9)

These materials are methacrylate copolymers containing spiropyranchromophores with a NO₂ terminal group attached to a methacrylatebackbone through a 6-carbon alkyl spacer. The general molecularstructure is shown in FIG. 6 . Spiropyran copolymers with a ratio of onespiropyran monomer to five methacrylate backbone units (1:5) and a ratioof one spiropyran monomer to nine methacrylate backbone units (1:9) havebeen shown to be useful. To the best of the inventor's knowledge, it hadnot been previously known prior to the inventor's discovery to use thesespiropyran copolymers to function as LC alignment layers, eitherwrite-once or photoswitchable.

FIG. 7 shows one example of how SPMA:MMA may be synthesized, with anoverall product yield of approximately 10%. [See S. Friedle and S. W.Thomas, Angew. Chem., Int. Ed. 49, 7968 (2010).]

3. SOMA:SOMA-PWA (1:1:6)

These materials (shown in FIG. 8 ) are also methacryate copolymers, butin this case contain two different spiroxazane methacrylate monomerchromophores copolymerized with unsubstituted methacrylate monomers in aratio of one pyridine-containing spiroxazane chromophore (SOMA) to onepiperidine-containing spiroxazane monomer (SOMA-p) to six unsubstitutedmethacrylate monomers. To the best of the inventor's knowledge, it hadnot been previously known prior to the inventor's discovery of theability of these spiroxazane copolymers to function as LC alignmentlayers, either write-once or photoswitchable.

FIG. 9 shows one example of how SOMA:SOMA-PMMA (1:1:6) may besynthesized, with an overall product yield of approximately 10%. [See T.H. Tan et al., Tetrahedron 61, 8192 (2005); and M. Fan et al., J. Chem.Soc. Perk. T 2, 1387 (1994).]

4. Suitability for Photoswitchable Alignment Layers

Materials from these three families of photoswitchable alignment layersare ideally suited for all-optical photoswitchable LC beam shapers forhigh power laser applications such as additive manufacturing due totheir exceptionally high laser damage thresholds at 1053 nm. The 1053nm, 1.4 ns damage thresholds of several examples of these materialscompared to existing data on other LC alignment materials (buffedlayers, write-once photoalignment layers, and photoswitchable alignmentlayers) are summarized in FIG. 10 . Notably, copolymer SPMA-MMA (1:5)exhibits the highest 1053 nm laser damage threshold values ever reportedfor any LC alignment layer composition which, at 90-100 J/cm², is nearlytwo orders of magnitude higher than that of the buffed polyimide coatingused in the matrix-addressed electro-optical LCOS-SLM and theelectrically-biased OALV devices of the prior art. Buffed alignmentlayers such as those used in the matrix addressed LCOS-SLM and OALV beamshapers of the prior art have the lowest 1053 nm laser damage thresholdsof the group (buffed polyimide is not shown, as its 1053 nm laser damagethreshold is <1 J/cm²). The ROLIC ROP 203/2CP, “Polymer 3” and LIA-01are write-once photoalignment materials, while the three PAAD materials,PESI-F, SPMA-MMA (1:5), and SOMA:SOMA-PMMA (1:1:6) are photoswitchable.

The ability to spontaneously align LC materials is an important factorin achieving beam shaper devices with high contrast ratios. FIG. 11shows the first three single-pixel LC devices fabricated using the newspiropyran (SPMA:MMA 1:5 and SPMA:MMA 1:9) and spiroxazane(SOMA:SOMA-p:MMA 1:1:6) photoswitchable polymers viewed under crossedpolarizers. Excluding fabrication defects, these photoswitchablealignment materials exhibit contrast, alignment uniformity, andwrite-state stability equivalent to or exceeding photoalignmentmaterials of the prior art. The white arrows in the photographs definethe alignment direction of the LC material in the device. All sampleswere photographed between crossed polarizers.

Liquid Crystal Materials for High Power Laser Beam Shaping

Several nematic LC materials were selected to explore the effect ofvarying degrees of π-electron delocalization and electron density ontheir damage thresholds. The aim was to provide baseline measurements onthe LIDT's of currently available LC's as a function of their chemicalstructure and extend the limited available knowledge on LC damagethresholds for nanosecond pulses at 1053 nm to both the sub-nanosecondand nanosecond pulse length regimes at 527 nm and 351 nm. Delocalizedπ-electrons are found in unsaturated (e.g., benzene-like) carbon ringswith double bonds, and their presence shifts the electronic absorptionedge toward longer wavelengths. Saturated compounds have carbon ringswith only single bonds, which essentially eliminate electrondelocalization and cause the absorption edge to be shifted towardshorter wavelengths. The wide range of LC materials evaluated are shownin Table 1. LC materials with the highest degree of π-electrondelocalization include the well-known cyanobiphenyl (two unsaturatedhydrocarbon rings) LC materials such as 5CB (4-pentyl-cyanobiphenyl orK-15) and the eutectic mixture E7. Compounds composed of bothunsaturated (benzene) and saturated (cyclohexane) rings including a60/40 mixture of two unsaturated phenyl benzoate ester compounds (PPMeOBand PPPOB) used on the OMEGA laser and the partially saturatedphenylcyclohexane-based mixture ZLI-1646 (Merck). Other materialsevaluated included a saturated alkyl LC mixture (Merck MLC-6601) and aperfluorinated alkyl LC mixture (Merck MLC-2037). Finally, a saturatedisothiocyanate LC compound, which includes some π-electrondelocalization within the isothicyanate N═C═S group) was tested. Laserinduced damage threshold values of commercially available LC compoundsand mixtures determined in 1988 (using nanosecond laser pulses at 1053nm) were re-evaluated to take into account significant improvements inpurity since that time [See “Liquid-Crystal Laser Optics: Design,Fabrication, and Performance,” S. D. Jacobs, K. A. Cerqua, K. L.Marshall, A. Schmid, M. J. Guardalben, and K. J. Skerrett, J. Opt. Soc.Am. B 5, 1962-1979 (1988)]. The initial work that compared anunsaturated LC compound, 5CB, to its saturated analog, ZLI-S-1185(4-octylcyanobicyclohexyl), is expanded [See T. Z. Kosc, S. Papernov, A.A. Kozlov, K. Kafka, and K. L. Marshall, and S. G. Demos,“Laser-Induced-Damage Thresholds of Nematic Liquid Crystals at 1 ns andMultiple Wavelengths,” presented at Laser Damage 2018, Boulder, Colo.,23-26 Sep. 2018].

TABLE 1 Absorp- tion Name Supplier Edge

1550C Dabrowski^(∧) 294 nm

MLC- 2037 Merck 306 nm

ZLI-1646 Merck 324 nm

PPMeOB/ PPPOB LLE* 345 nm

5CB EMB 377 nm

E7 EMB 385 nm ^(∧)Isothiocyanate compound synthesized by M. Dabrowski,University of Warsaw. *A 60/40 mixture of two phenyl benzoate estercompounds used on the OMEGA laser and synthesized at LLE.

In Table 1, materials are designated as saturated and unsaturated withthe symbol S or O, respectively, in the molecular structures in thefirst column. Note that the ZLI-1646 mixture contains contain compoundswith both saturated and unsaturated ring structures. Here, theabsorption edge is defined at T=98%.

Pulse Length Dependence at 1053 nm

The LIDT dependence at 1053 nm as a function of laser pulse duration wasinvestigated at six different pulse lengths: 600 fs, 2.5 ps, 10 ps, 50ps, 100 ps, and 1.5 ns. The 1-on-1 and N-on-1 LIDT values plotted as afunction of each material's UV-absorption edge (and therefore the linearabsorption cross section) are shown in FIG. 12 . The saturated materialshave an absorption edge<330-nm, and data for saturated and unsaturatedmaterials can be differentiated easily. The partially saturated LCmixture ZLI-1646 behaves more like a fully saturated material,suggesting that at least in this material, the unsaturated components,which one might expect to be the ‘weak links’, did not adversely affectthe LC mixture's performance. The LIDT values determined in this workfor three common commercial LC materials (E7, SCB, and ZLI-1646) werehigher than those determined for the same materials in 1988, which weattributed to advances in chemical purification processes applied tocommercial LC materials in general. Of notable significance are the dataat 1.5 ns, where the LIDT values of saturated LC's approach those ofbare fused silica.

The compounds are identified in FIG. 12(a), with brackets identifyingthe saturated, unsaturated, and mixed materials. The results suggestthat, in general, saturated materials exhibit a 2× to 4× higher LIDTthan unsaturated compounds independent of pulse length. The results alsoshow that, at pulse lengths≥50 ps, the N-on-1 LIDT exceeds the 1-on-1LIDT for most materials. This increase in LIDT with pre-exposure tolaser pulses, commonly referred to as laser “conditioning,” indicatesthe presence of a laser-induced material modification that leads toimproved materials performance. In contrast, for both saturatedmaterials and the unsaturated mixture PPMeOB/PPPOB at 10 ps, the N-on-1LIDT is lower than the 1-on-1 LIDT, an effect commonly referred to as“incubation.” Neither conditioning nor incubation is strongly observedat either 600 fs or 2.5 ps.

A clear pulse-length dependence emerges from the N-on-1 LIDT resultsplotted in FIG. 13 on a log-log scale and fit as a function of pulselength using τ^(x) power dependence, where x=0.5. Stuart showed that fordielectric materials the τ^(0.5) power scaling (though experimentallyobserved to vary between 0.3<x<0.6) is valid for pulse lengths greaterthan 20 ps, where thermal diffusion effects govern the damage-initiationprocess [See B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W.Shore, and M. D. Perry, “Nanosecond-to-femtosecond laser-inducedbreakdown in dielectrics,” Phys. Rev. B 53, 1749-1761 (1996)]. Defectsor defect states, in particular, absorb laser irradiation, which leadsto free electrons and ionization of material [See (1) M. D. Feit and A.M. Rubenchik, “Implications of nanoabsorber initiators for damageprobability curves, pulse length scaling, and laser conditioning,” Proc.SPIE 5273, 74-82 (2004); (2) C. W. Carr, J. B. Trenholme, and M. L.Spaeth, “Effect of temporal pulse shape on optical damage,” Appl. Phys.Lett. 90, 041110 (2007); and (3) G. Duchateau, M. D. Feit, and S. G.Demos, “Strong nonlinear growth of energy coupling during laserirradiation of transparent dielectrics and its significance for laserinduced damage,” J. Appl. Phys. 111, 093106 (2012)]. As pulse lengthsdecrease below 10 ps, multiphoton ionization starts to contribute toelectron production, and in the sub-picosecond range, multiphotonionization becomes the dominant process. The approximate τ^(0.5)dependence has also been observed in biological materials [See A.Oraevsky, L. B. Da Silva, A. Rubenchik, M. Feit, M. E. Glinsky, M.Perry, B. M. Mammini, W. Small IV, and B. C. Stuart, “Plasma mediatedablation of biological tissues with nanosecond-to-femtosecond laserpulses: Relative role of linear and nonlinear absorption,” IEEE J. Sel.Top. Quantum Electron. 2, 801-809 (1997)]. At this time, the fact thatLC LIDT's at pulse lengths<50 ps still follow the τ^(0.5) trendreasonably well appears coincidental. The fit for saturated materials isstronger (R²˜0.96), and the three samples with the lowest absorptionedges behave very similarly.

To better quantify the relative difference in damage thresholds betweensaturated and unsaturated LC materials, their value at each laser pulselength was normalized to the LIDT's of the cyanobiphenyl LC mixture E7,one of the most commonly used unsaturated LC mixture formulations.Results shown in FIG. 14 indicate that there is a difference in LIDTvalues between the two types of materials of about 2× for the shortest(600 fs) and longest (1.5 ns) pulse lengths tested. The dissimilarityincreases at intermediate pulse lengths (2.5 ps to 100 ps) with amaximum value greater than 3× at the 10-ps pulse length. Larger LIDTvariations (20% to 50%) were also observed for unsaturated LC materialsas compared to those for their saturated counterparts (5% to 11%). Thislarger variation in the LIDT of unsaturated materials is attributed totheir increased and varying amounts of π-electron delocalization.

The LIDT results were also examined as a function of the laser peakintensity at each pulse length. The results, shown in FIG. 15 , quantifythe difference in intensity at the LIDT observed between successive testpulse lengths for both saturated and unsaturated materials. FIG. 15shows that the intensity required to induce laser damage decreases withpulse length, underscoring the likely presence of multiple damagemechanisms that are driven by differing intensity requirements. Thevertical lines and associated quantification factors depict thedifference between the average N-on-1 intensities (at subsequent pulsedurations) required to induce damage in saturated and unsaturatedmaterials.

At the shortest pulse lengths, both types of materials undergo a similar˜3× reduction in damage threshold intensity between 600 ps and 2.5 ps.Similarly, as the pulse duration increases from 50 ps to 100 ps and from100 ps to 1.5 ns, the average damage intensity changes by similaramounts in each increment for both saturated and unsaturated materials(˜1.5× and ˜2.5×, respectively). However, around 10 ps, the damageintensity changes by differing amounts for the two materials types.Specifically, between 2.5 ps and 10 ps, the change in the damagethreshold intensity is lower for the saturated materials than for theunsaturated materials (1.7× and 2.6×, respectively). This difference isreversed between 10 ps and 50 ps, where the change in the damagethreshold intensity is 2.9× and 1.8× for the saturated and unsaturatedmaterials, respectively.

Wavelength Dependence at about 1 ns Pulse Duration

The excitation process is dependent on the electronic structure of thematerial and, as such, should depend strongly on the laser wavelength.Nematic LC materials were tested using nanosecond laser excitation at351-nm (third harmonic, 3ω) and 527-nm (second harmonic, 2ω) tocompliment the results obtained at the fundamental 1053-nm (1ω)wavelength presented above. This multiple-wavelength investigation aimsto probe the correlation between the electronic structure of eachmaterial and its laser-induced damage behavior via altering theexcitation photon energy.

The electronic excitation pathways in LC materials are generally knownand involve a singlet ground state (S₀) and excited singlet (S₁, S₂, . .. S_(n)) and triplet states. The time scale of the transition from thesinglet states to the corresponding triplet states during relaxation, orintersystem crossing, is typically >1 ns, which has been confirmed forseveral unsaturated LC compounds [See (1) F. H. Loesel, M. H. Niemz, J.F. Bille, and T. Juhasz, “Laser-induced optical breakdown on hard andsoft tissues and its dependence on the pulse duration: experiment andmodel,” IEEE J Quant Elect, 32 (10), 1717-1722 1996; and (2) A.Oraevsky, L. B. Da Silva, A. Rubenchik, M. Feit, M. E. Glinsky, M.Perry, B. M. Mammini, W. Small IV, and B. C. Stuart, “Plasma mediatedablation of biological tissues with nanosecond-to-femtosecond laserpulses: Relative role of linear and nonlinear absorption,” IEEE J. Sel.Top. Quantum Electron. 2, 801-809 (1997)]. Because the excitationleading to laser induced damage (breakdown) occurs during the laserpulse, transitions with lifetimes longer that the pulse duration (in ourcase ˜1 ns) will not have any effect on laser damage mechanisms.Consequently we consider only the transitions between the singletstates. The accordingly modified Jablonski energy diagram in FIG. 16describes the electronic structure in LC materials involving a singletground state (S₀) and excited singlet (S₁, S₂, . . . S_(n)) where theenergy levels are defined as multiples of the energy of a 1053 nm photonused in this study. Transmission measurements for each material providedinsight into which photon absorption order would be required to bridgethe energy gap from S₀→S₁.

The wavelengths designating the onset of linear absorption for each LCmaterial given in Table 1 are used as a guide to suggest the order ofphoton absorption required for the S₀→S₁ electronic state transition forunsaturated and saturated materials. Under 1053-nm laser irradiation,the unsaturated materials require three-photon absorption for the S₀→S₁transition, while the saturated materials require four-photonabsorption. This difference in the order of the absorption processrequired to generate excited-state electrons is captured clearly by thedifference in the damage threshold between the two types of materials,where the saturated materials have 2× to 3× higher damage thresholdacross all pulse lengths tested (FIGS. 12 and 14 ).

The LIDT results shown in FIG. 17 indicate that the laser-induced-damagethresholds under irradiation with 351-nm and 1-ns pulses follow theabsorption edge of the LC materials, a trend that is particularly clearfor the N-on-1 results. This behavior may be assigned in part to theorder of the excitation process. As depicted in the schematicrepresentation of the energy structure of the saturated and unsaturatedmaterials shown in FIG. 16 the highly unsaturated materials require1-photon absorption for the S₀→S₁ transition while saturated materialsrequire a 2-photon absorption process. This difference is potentiallyresponsible for the large difference in the LIDT (˜20×-50×, depending ondamage testing method) between the saturated and unsaturated materials.The results also show laser conditioning (N-on-1 LIDT>1-on-1 LIDT)occurs in saturated materials.

The LIDT results under irradiation with 527-nm and 1.2-ns pulses areshown in FIG. 18 . Based on the absorption characteristics of allmaterials, both unsaturated and saturated materials require 2-photonabsorption to populate the first excited state. Therefore, the strongdependence of LIDT on material saturation (˜10-15× difference) cannot beassigned to the S₀→S₁ transition cross sections. It is possible thatthis behavior arises from differences in the absorption cross sectionfor transition between excited states S₁→S₂. Specifically, it ispossible that the saturated materials require 2-photon absorption forthe S₁→S₂ transition, while unsaturated materials need only 1-photonabsorption. This difference in the electronic excitation process isdepicted in the schematic representation of the electronic energy leveldiagram shown in FIG. 16 . Previous studies of the excited stateabsorption spectrum in different LC materials revealed that the energyseparation between the first two excited states can be either higher orlower than the 527 nm photon energy, depending on the material [See (1)R. Sander, V. Herrmann, and R. Menzel, “Transient absorption spectra andbleaching of 4′-n-pentyl-4-cyanoterphenyl in cyclohexane—determinationof cross sections and recovery times,” J. Chem. Phys. 104, 4390-4395(1996); and (2) G. E. O'Keefe, J. C. De Mello, G. J. Denton, K. J.McEwan, and S. J. Till, “Transient excited-state absorption of theliquid crystal CB15 [4-(2-methylbutyl)-4-cyanobiphenyl] in its isotropicphase,” Liq. Cryst. 21, 225-232 (1996)]. Significant laser conditioningis once again observed in the saturated materials.

The LIDT results under irradiation with 1053-nm, 1.5-ns pulses wereshown previously in FIG. 12(a). The difference in the LIDT betweensaturated and unsaturated materials can be assigned to the change in theorder of the excitation process for the S₀→S₁ transition (3-photon vs4-photon absorption, respectively). This difference in the order of theabsorption process required to generate excited-state electrons isclearly captured by the difference in the LIDT between the two types ofmaterials. Although the difference in LIDT at 1053 nm between saturatedand unsaturated materials is 2× to 3× higher across all pulse lengthstested, these values are relatively small compared to the difference inLIDT between the two materials systems observed with 527 nm (˜15×). and351 nm pulses (˜50×).

What Was Learned from This Study

The results demonstrate that the LIDT values exhibit a strong dependenceon the incident laser wavelength, which indicates that damage initiationis sensitive to the energy separation between the ground and excitedstates. In order to deposit a sufficient amount of energy to initiatedamage, electrons might be excited to a level S_(n)≥S₂. Among allprobable pathways for the S₀→S_(n) transition, the one that involves thelowest order excitation processes (through existing intermediate states)is expected to be the dominant mechanism. In our system, this principleimplies that the electrons will first undergo the S₀→S₁ transition,followed by the S₁→S₂ transition, and then followed by possible singlephoton transitions to reach higher excited states (due to smaller energyseparations).

Upon excitation of electrons to the first excited state, additionalexcited-state absorption will require a lower-order absorption process,because the energy separation between S₁ and S_(n) states is smallercompared to that between S₀ and S₁ states [See (1) R. Sander, V.Herrmann, and R. Menzel, “Transient absorption spectra and bleaching of4′-n-pentyl-4-cyanoterphenyl in cyclohexane—determination of crosssections and recovery times,” J. Chem. Phys. 104, 4390-4395 (1996); and(2) G. E. O'Keefe, J. C. De Mello, G. J. Denton, K. J. McEwan, and S. J.Till, “Transient excited-state absorption of the liquid crystal CB15[4-(2-methylbutyl)-4-cyanobiphenyl] in its isotropic phase,” Liq. Cryst.21, 225-232 (1996)]. This smaller energy separation, in turn, yields ahigher absorption cross section for excited-state absorption (ESA) andis therefore critical in the context of laser damage. Because thelifetime of the Si state is of the order of 1 ns or longer, in thiswork, the excited electrons do not return to the ground state during thelaser pulse [See (1) R. Sander, V. Herrmann, and R. Menzel, “Transientabsorption spectra and bleaching of 4′-n-pentyl-4-cyanoterphenyl incyclohexane—determination of cross sections and recovery times,” J.Chem. Phys. 104, 4390-4395 (1996); and (2) G. E. O'Keefe, J. C. DeMello, G. J. Denton, K. J. McEwan, and S. J. Till, “Transientexcited-state absorption of the liquid crystal CB15[4-(2-methylbutyl)-4-cyanobiphenyl] in its isotropic phase,” Liq. Cryst.21, 225-232 (1996)]. Consequently, ESA is a more effectiveenergy-deposition mechanism, but is limited by the availableexcited-state electron population. Therefore, two general governingmechanisms that contribute to absorption of energy by the laser pulsecan be considered: (a) direct absorption by ground-state electrons and(b) absorption by excited-state electrons involving only the singletstates. The excited-state electrons can, in principle, undergo multipleabsorption cycles by either reaching the higher excited state (S₂) andreturning to the Si state during the laser pulse to repeat the processor continuing with additional absorption toward higher excited states(S₂→S_(m)).

Laser-induced damage experiments exploring pulse length scaling provideinsight into energy-deposition mechanisms. Of particular interest is thechange in the normalized LIDT between the two types of materials at 10ps (FIG. 14 ), which transitions from a two-fold to a three-foldincrease before returning to initial values at longer pulse lengths.This change is also captured in FIG. 15 in the same pulse-length regimeby the very different factors between LIDT at successive pulse lengths.The behavior observed in FIGS. 14 and 15 in the range from 3 to 50 pslikely arises from the complex interplay between energy-depositionmechanisms, as well as the temporal behavior of the ESA cross section.Specifically, O'Keefe showed that for CB15 (an unsaturated cyanobiphenylidentical in structure with 5CB except that it has a chiral alkylterminal group instead of the straight-chain alkyl group found on 5CB)the ESA cross section can change rapidly within time scales of the orderof 10 to 50 ps, which could directly impact the average efficiency (rateof energy deposition) of ESA as a function of the laser pulse length[See G. E. O'Keefe, J. C. De Mello, G. J. Denton, K. J. McEwan, and S.J. Till, “Transient excited-state absorption of the liquid crystal CB15[4-(2-methylbutyl)-4-cyanobiphenyl] in its isotropic phase,” Liq. Cryst.21, 225-232 (1996)].

To better capture the relative difference in the measured LIDT atdifferent wavelengths and their relationship to the electronic structureof each type of material (summarized in FIG. 12 ), the average N-on-1LIDT results for both saturated and unsaturated materials at allwavelengths are shown in FIG. 19 . Comparison of the 351-nm and 527-nmresults shows a difference between LIDT values of ˜5× for unsaturatedmaterials and only of ˜1.5× for saturated materials. This behavior canbe anticipated from the type of absorption order required for electronsto undergo the S₀→S₁ transition. Specifically, unsaturated materialsrequire both linear absorption at 351 nm and 2-photon absorption at 527nm, while for saturated materials, 2-photon absorption is necessary topopulate the first excited state at both wavelengths. This keydifference in the electronic excitation process is reflected in thecorresponding difference in the LIDT values.

Comparing LIDT results obtained under 351-nm and 1053-nm excitation, thedifference in LIDT for unsaturated materials is ˜150× but only ˜8× forsaturated materials. The dramatic variation in LIDT differences for thetwo material types is arguably related to the different order of theabsorption process required for the S₀→S₁ transition. The order changesfrom linear absorption to a 3-photon absorption process in unsaturatedmaterials, while for saturated materials a nonlinear process is requiredat both wavelengths (2-photon and 4-photon processes for 351-nm and1053-nm excitation, respectively).

Laser conditioning (N-on-1 LIDT>1-on-1 LIDT) was only observed under asubset of conditions: (a) both material types at 50 ps, 100 ps, and 1.5ns at 1053 nm and (b) saturated materials at 527 nm and 351 nm. The LIDTresults for unsaturated LC's (5CB, E7 and the PPMeOB/PPPOB mixture) andsaturated LC's (1550C, MLC-2037, and the partially saturated ZLI-1646)were averaged at each wavelength and plotted as a function of the pulselength in FIG. 20 . The subset of data associated with laserconditioning is noted.

The highlighted data associated with observations of laser conditioningshown in FIG. 20 are found in the upper right quadrant with intensitieshigher than ˜5 GW/cm² and pulse duration longer than 10 ps. Thisobservation may suggest that a threshold intensity exists for laserconditioning. Unsaturated compounds have LIDT values lower that thisconditioning threshold intensity (<5 GW/cm²) at both 351 nm and 527 nm.Results also suggest that laser conditioning is limited to time scalesgreater than 10 ps. There are two scenarios for this limitation. In onecase, conditioning involves a two-step process with a characteristicrise time (on the order of 10 ps) associated with promotion of electronsto an excited state, followed by transition to an intermediate statewhere further excitation can facilitate conditioning. In the secondcase, the laser conditioning mechanism is dictated by the time scale ofphotochemically-induced reaction kinetics (e.g. volatilization ofimpurities or photochemically-induced reaction of the LC molecules withintrinsic or extrinsic impurities, breakdown products, or with eachother to form more stable compounds (eg. oxygen bridge formation inbiphenyls).

In summary, this study demonstrate that the LIDT values show strongdependence on wavelength and electronic structure, which in turnprovides information about the excitation pathways leading to laserinduced damage. Experimental data suggest that key components in thelaser-induced damage mechanisms in LC's involve a complex interplay ofboth multiphoton absorption and excited-state absorption, where theirrelative contributions vary with both pulse length and wavelength. Ingeneral, saturated materials are shown to provide a higher LIDT at allwavelengths and pulse lengths.

The invention claimed is:
 1. A beam shaping system, comprising: (a) a liquid crystal beam shaper, the beam shaper comprising liquid crystals and a photoswitchable alignment layer, the photoswitchable alignment layer comprising at least one material from the group of PESI-F, SPMA:MMA 1:5, SPMA:MMA 1:9, or SOMA:SOMA-p:MMA 1:1:6; and (b) an optical writing and erasing sub-system configured to write, erase, and rewrite a plurality of optical patterns in the photoswitchable alignment layer; wherein PESI-F is a poly(esterimide) comprising the repeat unit:

wherein SPMA:MMA 1:5 and SPMA:MMA 1:9 are copolymers comprising the repeat unit:

wherein n is 5 or 9, respectively; wherein SOMA:SOMA-p:MMA 1:1:6 is a copolymer comprising the repeat unit:

wherein n is
 6. 2. The beam shaping system of claim 1, wherein the photoswitchable alignment layer possesses an N-on-1 laser induced damage threshold of 80-100 J/cm² at 1053 nm.
 3. The beam shaping system of claim 1, wherein at least some liquid crystals of the beam shaper comprise at least one of partially saturated liquid crystals, fully saturated liquid crystals, partially fluorinated liquid crystals, or perfluorinated liquid crystals.
 4. The beam shaping system of claim 1, wherein at least some liquid crystals of the beam shaper comprise at least one of a phenylcyclohexane, a perfluorinated material, or saturated cyclohexane rings.
 5. The beam shaping system of claim 1, wherein the liquid crystal beam shaper comprises an all-optical liquid crystal beam shaper.
 6. The beam shaping system of claim 5, wherein the liquid crystal beam shaper comprises: (i) a first transparent glass substrate comprising a first coating on an inner surface of the substrate, the first coating comprising the photoswitchable alignment layer; (ii) a second transparent glass substrate comprising a second alignment coating on an inner surface of the substrate, the second alignment coating comprising either a buffed alignment layer or a layer that has been permanently oriented using polarized UV light; (iii) the liquid crystals of the beam shaper located between the first and second substrates; and (iv) a polarizer.
 7. The beam shaping system of claim 6, wherein the beam shaping system is configured such that writing an optical pattern into the photoswitchable alignment layer causes a localized change in configuration of the liquid crystals, such that a laser beam passing through the liquid crystal beam shaper undergoes a localized change in polarization state.
 8. The beam shaping system of claim 7, wherein the polarizer is configured to pass or reject portions of the laser beam depending on the localized change in polarization state.
 9. The beam shaping system of claim 6, wherein the first and second transparent glass substrates comprise fused silica.
 10. The beam shaping system of claim 9, wherein the second alignment coating comprises an alignment coating having a fixed alignment state.
 11. The beam shaping system of claim 10, wherein the second alignment coating comprises at least one of a poly(N,N′-hexamethyleneadipinediamide), a cinnamate photopolymer, a coumarin-based photoalignment layer material, or an azobenzene photoswitchable alignment layer.
 12. The beam shaping system of claim 6, wherein the optical writing and erasing sub-system comprises: (i) a UV light source and a spatial light modulator configured to write an optical pattern in the photoswitchable alignment layer; or (ii) a raster-scanned UV laser source configured to write the optical pattern in the photoswitchable alignment layer.
 13. The beam shaping system of claim 12, wherein the optical writing and erasing sub-system is configured to erase the written optical pattern by application of UV light having a different polarization than the UV light used to write the optical pattern, or by application of visible light.
 14. A beam shaping system, comprising: (a) a liquid crystal beam shaper, the beam shaper comprising liquid crystals and a photoswitchable polymer command surface; and (b) an optical writing and erasing sub-system; the photoswitchable polymer command surface and the optical writing and erasing sub-system configured to write, erase, and rewrite a plurality of optical patterns in the photoswitchable alignment layer; wherein at least one of the components in a beam path of the liquid crystal beam shaper have an N-on-1 laser induced damage threshold using a small beam damage testing configuration exceeding: (i) 40 J/cm² at 1053 nm and 1500 ps pulse width; or (ii) 5 J/cm² at 1053 nm and 100 ps pulse width; or (iii) 1 J/cm² at 1053 nm and 10 ps pulse width; or (iv) 0.8 J/cm² at 1053 nm and 0.6 ps pulse width.
 15. The beam shaping system of claim 14, wherein at least some liquid crystals of the beam shaper comprise at least one of partially saturated liquid crystals, fully saturated liquid crystals, partially fluorinated liquid crystals, or perfluorinated liquid crystals.
 16. The beam shaping system of claim 15, wherein the liquid crystal beam shaper comprises an all-optical liquid crystal beam shaper.
 17. A beam shaping system, comprising: (a) an all-optical liquid crystal beam shaper, the beam shaper comprising: (i) a first transparent glass substrate comprising a first coating on an inner surface of the substrate, the first coating comprising a photoswitchable alignment material comprising at least one of a SPMA:MMA 1:5, SPMA:MMA 1:9, or a SOMA:SOMA-p:MMA 1:1:6 material; (ii) a second transparent glass substrate comprising a fixed alignment coating on an inner surface of the substrate; (iii) liquid crystals between the first and second glass substrates, wherein at least some liquid crystals of the beam shaper comprise at least one of a phenylcyclohexane, a perfluorinated material, or saturated cyclohexane rings; and (iv) a polarizer; and (b) an optical writing and erasing sub-system configured to write, erase, and rewrite a plurality of optical patterns in the first coating; wherein SPMA:MMA 1:5 and SPMA:MMA 1:9 are copolymers comprising the repeat unit:

wherein n is 5 or 9, respectively; wherein SOMA:SOMA-p:MMA 1:1:6 is a copolymer comprising the repeat unit:

wherein n is
 6. 18. The beam shaping system of claim 17, wherein the beam shaping system is configured such that writing an optical pattern into the first coating causes a localized change in configuration of the liquid crystals, such that a laser beam passing through the liquid crystal beam shaper undergoes a localized change in polarization state.
 19. The beam shaping system of claim 18, wherein the polarizer is configured to pass or reject portions of the laser beam depending on the localized change in polarization state.
 20. The beam shaping system of claim 17, wherein the optical writing and erasing sub-system comprises: (i) a UV light source and a spatial light modulator configured to write an optical pattern in the first coating; or (ii) a raster-scanned UV laser source configured to write the optical pattern in the first coating.
 21. The beam shaping system of claim 1, wherein at least one of the components in a beam path of the liquid crystal beam shaper have an N-on-1 laser induced damage threshold using small beam damage testing configuration exceeding: (i) 40 J/cm² at 1053 nm and 1500 ps pulse width; or (ii) 5 J/cm² at 1053 nm and 100 ps pulse width; or (iii) 1 J/cm² at 1053 nm and 10 ps pulse width; or (iv) 0.8 J/cm² at 1053 nm and 0.6 ps pulse width.
 22. The beam shaping system of claim 17, wherein the beam shaping system is configured to shape a laser beam of a laser additive manufacturing system.
 23. A laser additive manufacturing system, comprising: (a) a laser beam source configured to generate a laser beam for additive manufacturing; and (b) a beam shaping system configured to repeatedly vary a profile of the laser beam, the beam shaping system comprising: (i) an all-optical, optically addressable beam shaper, comprising liquid crystals and a photoswitchable alignment layer; and (ii) an optical writing and erasing sub-system configured to repeatedly write and erase a plurality of optical patterns in the photoswitchable alignment layer to cause a localized change in alignment of the liquid crystals.
 24. The additive manufacturing system of claim 23, wherein the liquid crystals comprise at least one of partially saturated liquid crystals, fully saturated liquid crystals, partially fluorinated liquid crystals, or per-fluorinated liquid crystals.
 25. The additive manufacturing system of claim 23, wherein the liquid crystals comprise at least one of a phenylcyclohexane, a per-fluorinated material, or saturated cyclohexane rings.
 26. The additive manufacturing system of claim 24, wherein the beam shaper comprises: (i) a first transparent glass substrate and a first coating on an inner surface of the substrate, the first coating comprising the photoswitchable alignment layer; (ii) a second transparent glass substrate and a second alignment coating on an inner surface of the substrate, the second alignment coating having a fixed alignment state; (iii) the liquid crystals of the beam shaper located between the two alignment layers and substrates; and (iv) a polarizer configured to reflect a first polarization state and transmit a second polarization state that is complimentary of the first polarization state.
 27. The additive manufacturing system of claim 26, wherein the first and second coatings are electrically non-conductive.
 28. The additive manufacturing system of claim 23, wherein the liquid crystals comprise an absorption edge of less than 330 nm.
 29. The additive manufacturing system of claim 28, wherein the photoswitchable alignment layer comprises at least one of a PESI-F, SPMA:MMA 1:5, SPMA:MMA 1:9, or a SOMA:SOMA-p:MMA 1:1:6 material; wherein PESI-F is a poly(esterimide) comprising the repeat unit:

wherein SPMA:MMA 1:5 and SPMA:MMA 1:9 are copolymers comprising the repeat unit:

wherein n is 5 or 9, respectively; wherein SOMA:SOMA-p:MMA 1:1:6 is a copolymer comprising the repeat unit:

wherein n is
 6. 30. The additive manufacturing system of claim 23, wherein at least one of the components in a beam path of the beam shaper have an N-on-1 laser induced damage threshold using small beam damage testing configuration exceeding: (i) 40 J/cm² at 1053 nm and 1500 ps pulse width; or (ii) 5 J/cm² at 1053 nm and 100 ps pulse width; or (iii) 1 J/cm² at 1053 nm and 10 ps pulse width; or (iv) 0.8 J/cm² at 1053 nm and 0.6 ps pulse width. 